The conventional contact image sensors may be classified into two types: photoconductor type and photodiode type. In the photoconductor type image sensors, their photoconductivity is utilized and a current proportional to the voltage applied can be obtained. However, they are disadvantageous in that the response speed is slow. Accordingly, the photodiode type image sensors, in which a layer of amporphous silicon carbide or the like is used as a blocking layer and in which the rectifying effect of the diode is utilized so that carriers photoelectrically generated alone can afford the output current, have become a focus of attention.
However, while the photodiode type is advantageous in that the bright/dark current ratio is greater as compared with the photoconductor type, the photodiode type still has a problem to be solved. Thus, the photocurrent intensity is limited to a level lower than 1 when expressed in terms of quantum efficiency. Accordingly, charge accumulation and amplification, for instance, are required, presenting difficulties from the circuit viewpoint.
It is an object of the invention to provide a photosensor in which the dark current is slight and the bright current is fairly strong.